The present invention relates to solid and liquid compositions containing particles of highly fluorinated ion exchange polymer having sulfonate functional groups, a process for making such compositions, and products made using such compositions.
Liquid compositions of perfluorinated ion exchange polymers are known for use in the manufacture and repair of ion exchange membranes, for membrane coatings containing conductive or nonconductive particles, and for many other uses. While such compositions are sometimes referred to as solutions, the compositions are generally recognized as being dispersions of polymer particles.
U.S. Pat. No. 4,433,082 to Grot discloses a process for preparing such liquid compositions containing perfluorinated ion-exchange polymers having sulfonic acid groups or a salt thereof and having equivalent weights of 1025 to 1500. The medium preferred for use in U.S. Pat. No. 4,433,082 contains 20 to 90% by weight of water and 10 to 80% by weight of an organic compound such as a lower alcohol. While U.S. Pat. No. 4,433,082 indicates that liquid compositions can be prepared using water only, no useful processes for making liquid compositions without alcohols are disclosed.
Compositions made in accordance with U.S. Pat. No. 4,433,082 and containing water together with one or more lower alcohols are sold commercially under the trademark NAFION® by E.I. du Pont de Nemours and Company. Currently, the commercial compositions can contain up to 10% by weight of a perfluorinated ion-exchange polymer having sulfonic acid groups in a medium of approximately 30-60% by weight water, 15-30% by weight 1-propanol, 15-30% by weight 2-propanol, and less than 10% by weight (total) of miscellaneous components consisting of methanol, mixed ethers and other volatile organic compounds (VOC's). A typical commercial composition contains a nominal 5% by weight of the perfluorinated ion-exchange polymer having sulfonic acid groups in a medium of approximately 45% by weight water, 22% by weight 1-propanol, 22% by weight 2-propanol, 3% by weight methanol and 3% of mixed ethers and other VOC's.
For many uses, the alcohol in these compositions is undesirable. For example, known compositions are often used in the manufacture of electrodes containing catalyst particles for electrochemical cells such as fuel cells. Exposure of alcohol vapors to the catalyst particles can cause undesirable side reactions and can even be a fire hazard. In general, the presence of an alcohol means that the alcohol or its decomposition products will be released into the atmosphere when the composition is used. Releases of VOC's result, not only in the loss of the compounds, but are subject to reporting requirements and limits imposed by environmental authorities. Recovery systems can be employed but they generally require a large investment, are expensive to operate, and may not be cost effective, particularly for small scale operations.
The alcohol in known compositions can be partially or entirely removed to produce a composition which contains less alcohol or essentially only water by processes such as vacuum distillation. However, such compositions are expensive because of the additional processing steps. In addition, the problems relating to alcohol release or recovery are associated with the process used to remove alcohol from the compositions.
Nonaqueous compositions containing alcohol or another organic medium with little or no water content are also desired for some applications. While known compositions containing a mixture of water and alcohol can be converted to alcohol only compositions by processes such as azeotropic distillation, these processes are time consuming and expensive. Nonaqueous compositions in a nonaqueous media other than alcohol have are not typically been available due to the difficulty in manufacture.
Moreover, commercially-available compositions typically have a low concentration of polymer (in the range of about 5% by weight) and are unsuitable for applications where higher concentrations are desired. For example, when coatings are made, it is often necessary to use processes which employ repetitive applications of the composition to make the desired coating thickness and such processes are usually complicated, time consuming and costly.